Feedback-comparison positioning systems are well known and are commonly used in many applications. In such systems a position command is continuously and intermittently compared to an actual position signal and an error signal is generated which is related to the difference between the two. This error signal is typically applied to a feed control device such as a motor which responds in proportion to the error signal.
One application for such a system is an industrial milling or grinding machine wherein a tool must be displaced relative to a workpiece in order to perform a cutting, welding, contouring or metal working operation. In such applications the need often arises to produce a sequence of machine movements wherein the different end positions of a movable member are specified in a nonprogressive sequence; that is, one in which there is no constant arithmetic relationship between the sequence of end positions and, in fact, the member may be required to move in both forward and reverse directions to reach the commanded end positions. Moreover, there is a need for the capability for specifying a plurality of such end positions in advance of the execution of a sequence of movements; i.e., the position commands must be definable in advance. One way to accomplish this is to prepare a programming medium such as a punched tape or card in which the sequence of end positions are specified. The problem with this approach, however, lies in the difficulty of accomplishing adjustments and variations in the program.
The present invention provides a means to perform a sequence of progressive or non-progressive moves in a highly precision fashion and to permit the end points of such moves to be programmed in advance, yet remain easily changed even during actual selection of a given end point.